1. Field of the Invention
The present invention relates to a metallic porous body comprising a metallic framework for alkaline batteries, such as a nickel-cadmium battery, a nickel-hydrogen battery, and a nickel-zinc battery; a method for manufacturing the metallic porous body; and a current collector having the metallic porous body.
2. Description of the Background Art
Sponge-like metals and sintered metals have been widely used as the current collector for alkaline batteries. Sponge-like metals comprise a metallic framework having a three-dimensional network with a continuous-pore structure formed by linking sub-stantially polyhedral cells. Because they have a porosity as high as about 90 to about 98%, they can increase the filling amount of an active material per unit volume, offering a current collector having high capacity density (mAh/cc) as an advantageous feature.
Sintered metals have a porosity no more than about 85%, which is inferior to sponge-like metals. However, sintered metals have pore diameters as small as several micrometers, allowing a large current to flow. As a result, they can offer a high-output current collector as an advantageous feature.
Development of new technologies such as hybrid electric cars has been strongly required in recent years to cope with environmental problems and energy problems. Batteries for such electric cars need a high-output current collector having, for instance, a sintered metal.
However, it is difficult to fill an active material into sintered metals by the paste method. Consequently, the reduced-pressure impregnation method must be applied repeatedly. This means that filling of the active material must be carried out by batch processing rather than by continuous processing, posing a problem of filling-cost increase.
In order to overcome this drawback in sintered metals, an idea has been pro-posed to offer a current collector that is suitable for high-power use in which a large current must be allowed to flow. The idea is to reduce the diameter of substantially polyhedral cells in a sponge-like metal without losing the sponge-like metal""s advantages of high porosity and high capacity density.
However, a reduction in the diameter of the substantially polyhedral cells by conventional techniques entails a concurrent reduction in the size of the windows of internally existing cells that can be seen through the opening of the substantially polyhedral cells on a surface of the sponge-like metal.
This reduction in the size of the windows has posed problems in that the windows are easily clogged in the filling process of an active material and that it is difficult to fill an active material uniformly into the sponge-like metal. These problems have caused serious problems of cracking in the subsequent rolling process and a reduction in the final battery capacity.
An object of the present invention is to offer a metallic porous body that maintains high capacity density, that is free from clogging in the filling process of an active material, that allows the active material to fill the body uniformly even into the deep inside, and that thereby allows a large current to flow; and to offer a method of manufacturing the metallic porous body.
Another object of the present invention is to offer a current collector for batteries having a high output and large battery capacity.
After intensive studies to solve the above-described problems, the present inventors found that when the xe2x80x9ccell diameterxe2x80x9d and xe2x80x9cwindow diameterxe2x80x9d in a sponge-like metal are controlled to fall within their respective specified ranges, a high output can be obtained without losing the sponge-like metals advantages of high porosity and high capacity density, and completed the present invention.
In the present invention, the terms xe2x80x9ccell diameterxe2x80x9d and xe2x80x9cwindow diameterxe2x80x9d are defined as follows when used in relation with a metallic porous body, such as a sponge-like metal, or a plastic porous body with a framework, where the metallic porous body and plastic porous body have a continuous-pore structure formed by linking substantially polyhedral cells: xe2x80x9ccell diameterxe2x80x9d: the longest diagonal of the substantially polygonal opening of a substantially polyhedral cell on a surface of a metallic or plastic porous body (see FIG. 1), and xe2x80x9cwindow diameterxe2x80x9d: the longest diagonal of the substantially polygonal window of an internally existing, substantially polyhedral cell that can be seen through the above-mentioned opening of another substantially polyhedral cell on a surface of a metallic or plastic porous body (usually, a plurality of such windows can be seen through an opening)(see FIG. 1).
The metallic porous body of the present invention comprises a metallic framework having a three-dimensional network with a continuous-pore structure formed by linking substantially polyhedral cells. The substantially polyhedral cells have an average cell diameter of about 200 to about 300 xcexcm and an average window diameter of about 100 to about 200 xcexcm.
The metallic porous body of the present invention is suitable for a high-power current collector that allows a large current to flow, because the metallic porous body has a small average cell diameter.
In addition, the metallic porous body of the present invention has a larger average window diameter than conventional metallic porous bodies that have a similar average cell diameter. Consequently, the metallic porous body of the present invention can be filled with an active material uniformly with high density, enabling a further increase in the battery capacity without losing the sponge-like metals advantages of high porosity and high capacity density.
The metallic porous body of the present invention can be obtained by the following methods, for instance:
(a) First, a plastic porous body is provided that has an average cell diameter of about 200 to about 300 xcexcm and an average window diameter of about 100 to about 200 xcexcm. Second, a conductive layer is formed on a surface of the framework of the plastic porous body by electroless plating to produce a conductive porous body having a resistivity of about 1 kxcexa9xc2x7cm or less. Finally, a continuous metalplated layer is formed on a surface of the conductive layer by electroplating, with the conductive porous body serving as the cathode. (b) First, a plastic porous body is provided that has an average cell diameter of about 200 to about 300 xcexcm and an average window diameter of about 100 to about 200 xcexcm. Second, a conductive layer is formed on a surface of the framework of the plastic porous body by applying a solution of binder resin containing a carbon powder to produce a conductive porous body having a resistivity of about 5 kxcexa9xc2x7cm or less. Finally, a continuous metal-plated layer is formed on a surface of the conductive layer by electroplating, with the conductive porous body serving as the cathode.
It is desirable to remove the plastic porous body by heat treatment after the metal-plated layer is formed.
In the present invention, an electrical resistance for calculating the foregoing resistivity is measured by the configuration shown in FIG. 2. A conductive porous body 1 with a size of 10xc3x97150 mm is provided as a test sample. Two potential terminals 2A and 2B are placed 100 mm apart on the sample. Two current terminals 3A and 3B are placed outside the potential terminals as shown in FIG. 2. A potentiometer 4 is connected to the potential terminals to measure the potential drop when a current of 1A is injected through the current terminals. A reading is usually taken 1 to 5 seconds after the current injection. A total weight of 516.83 g is applied to the sample by the four terminals.
The manufacturing method (a) above is advantageous because it can further reduce the resistivity of the conductive layer, and it facilitates the formation of a continuous, uniform metal-plated layer in the electroplating process.
The first method for manufacturing the metallic porous body of the present invention is as follows: First, a conductive layer is formed on a surface of the framework of a plastic porous body by electroless plating to produce a conductive porous body having a resistivity of about 1 kxcexa9xc2x7cm or less. Then, a continuous metal-plated layer is formed on a surface of the conductive layer by electroplating, with the conductive porous body serving as the cathode.
The first method can further reduce the resistivity of the conductive layer. It is preferable, and still possible, to reduce it as low as about 100xcexa9xc2x7cm or less. Such a method enables the formation of a metal-plated layer with uniform thickness in the thickness direction of the plastic porous body, even with an extremely fine structure of the metallic porous body of the present invention.
The second method for manufacturing the metallic porous body of the present invention is as follows: First, a conductive layer is formed on a surface of the framework of a plastic porous body by applying a solution of binder resin containing a carbon powder to produce a conductive porous body having a resistivity of about 5 kxcexa9xc2x7cm or less. Then, a continuous metal-plated layer is formed on a surface of the conductive layer by electroplating, with the conductive porous body serving as the cathode.
The second method can form an extremely fine structure of the metallic porous body of the present invention by a simple method such as to apply a solution of binder resin containing a carbon powder, enabling a reduction in the manufacturing cost.
The battery-use current collector of the present invention specifically has the metallic porous body of the present invention.
The battery current collector of the present invention has a metallic porous body that not only can increase the battery capacity but also enables the dense, uniform filling of an active material. Therefore, the current collector is suitable for use in batteries that need to have a high output and large capacity, such as those used in electric cars.
As described above, the present invention offers a metallic porous body that enables the dense, uniform filling of an active material without decreasing large battery capacity, which is an important feature of a metallic porous body such as a sponge-like metal. Such a metallic porous body can offer a current collector having large capacity and a high output.